Machines such as, for example, wheel loaders, dozers, backhoes, dump trucks, and other heavy equipment typically utilize automatic transmissions having a plurality of gears that are selectively engaged in predetermined combinations to produce desired gear output ratios. In an automatic transmission, the selective engagement of the plurality of gears is often performed according to preprogrammed shift points that are based on full-throttle engine speeds. At full-throttle engine speeds, the power output of an associated power train is maximized.
One way to improve fuel economy during full-throttle shifting is to improve the efficiency of the torque converter by utilizing a lockup clutch. A torque converter efficiency generally decreases as the converter speed approaches approximately 90 percent of the output speed of the associated engine due to the characteristics of the fluid in the torque converter. Engaging a lockup clutch directly links the pump of the torque converter to the turbine of the torque converter, thereby effectively creating a mechanical coupling and improving the efficiency of the torque converter.
Another method used to improve fuel economy is to utilize shift points based on part-throttle engine speeds when performing operations not requiring a maximum power output from the power train. Such a strategy may improve fuel economy because fuel consumption at part-throttle speeds is less than fuel consumption at full-throttle speeds. One example of a part-throttle shifting system can be found in U.S. Pat. No. 5,846,161 (the '161 patent) issued to Hosseini et al. on Dec. 8, 1998. The '161 patent discloses a control system that regulates shifting events based on signals indicative of an engine speed, a torque converter output speed, and a currently active gear ratio. The shift points at which shifting events occur are preprogrammed full-throttle shift points. However, an operator can adjust a minimum speed setting to use part-throttle shift points. The minimum speed setting is the lowest engine speed at which a particular shift event is permitted to occur.
Although the shifting strategy disclosed in the '161 patent may utilize part-throttle shifting, any improvement to fuel economy may be limited. In particular, the shifting strategy employed by the '161 patent does not utilize a lockup clutch in conjunction with either part-throttle or full-throttle shift strategy. Without the utilization of the lockup clutch, any improvement to the efficiency of the torque converter and the fuel economy of the power train may be limited.
The disclosed system is directed to overcoming one or more of the problems set forth above.